<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    .outer{
      width: 1000px;
      height: 900px;
      background-color: #888888;
      margin: 0 auto;
      display: flex;
      box-sizing: border-box;
      display: flex;

      justify-content: center;
      justify-content: space-between;
      justify-content: space-evenly;
      /*justify-content: flex-start;*/
      /*justify-content: flex-end;*/
      justify-content: space-around;
    }
    .inner{
      width: 200px;
      height: 200px;
      background-color: skyblue;
      border: 1px solid black;
    }
  </style>
</head>
<body>
<div class="outer">
  <div class="inner inner1">1</div>
  <div class="inner inner2">2</div>
  <div class="inner inner3">3</div>

</div>
</body>
</html>